One of the principal problems involved in designing horizontal axis wind turbines is wind shear, which is the variation of wind velocity with height above ground level. Wind velocities tend to increase with altitude due to aerodynamic surface drag and the viscosity of air. As a result, turbine blades at the top of a rotary path experience higher wind velocities than blades at the bottom of the rotary path. If this vertical wind velocity gradient is not addressed in the design of the wind turbine, then it will subject the components of the wind turbine to damaging stresses during rotary operation.
In addition to wind shear due to natural differences in wind velocity with altitude, wind shear can also be induced by improper alignment of the main shaft axis, i.e., not placing the axis at the optimal angle with respect to wind direction. Most often, improper alignment results from changes in wind direction. If there is no wind shear, the rotor axis (the axis around which the blades rotate) should face directly into the wind so that all blades will experience the same wind speed. If however, the main shaft axis is aligned obliquely to the wind in one direction, blades at the top of the rotation move into the wind, and blades at the bottom of the rotation will move with the wind. This will cause blades at the top of the rotation to experience a greater effective wind speed than blades at the bottom. Conversely, if the orientation of the main shaft axis is oblique to the wind in the opposite direction, blades at the bottom of the rotation will experience a greater effective wind speed than those at the top. Other sources of wind shear include wind turbulence, shadowing from a turbine's own tower as well as shadowing from neighboring turbines (e.g., for wind turbines located within a wind farm). Additionally, for turbines deployed in aqueous environments, there are significant differences in the flow rate of the water. Typically water at the top of a flowing stream runs faster than water at the bottom of the stream.
The lift generated by turbine blades during rotation is applied both in the direction of rotation and in a backward direction. Forces applied in the direction of rotation are designated as in-plane forces and forces applied in a backward direction are designated as out-of-plane forces. Because of this, wind shear will cause more backward force to be applied to blades experiencing the greater effective wind speed. With a rigid hub, the unbalance in backward forces creates a cyclical stress on the blades and bearings that can cause excessive wear and maintenance problems, and can shorten the useful life of the wind turbine generator.
One approach for addressing the problems associated with wind shear involves use of a hub incorporating a “teeter pin” that enables a turbine rotor to pivot back-and-forth like a playground seesaw. This back-and-forth rotation results in balancing of the torque on the blades around a teeter axis because blades experiencing higher wind velocity move with the wind and blades experiencing lower wind velocity move into the wind. Such teeter pins are useful as applied to two-bladed wind turbines, as they allow the upper blade to tilt backward while the lower blade tilts forward. Thus, the teetering motion of a two-bladed wind turbine tends to equalize the effective wind speeds for both blades, thereby maintaining a more constant tip speed ratio. The pivotal movement enabled by teeter pins, however, is inadequate to compensate for wind shear in turbines having three or more blades. This is because teetering is limited to one blade moving forward and the other moving backward in an equal and opposite manner across a single rotating teeter axis.
Another approach for addressing problems associated with wind shear involves use of a ball-and-socket hub that enables teetering of a turbine rotor with three or more blades, such as described in U.S. Pat. Nos. 8,708,654 and 9,194,366. A three-bladed design is widely considered to be the optimal configuration for large horizontal axis wind turbines. A paper authored by the inventor of this application and published in the Journal of Solar Energy Engineering 137(3) in June 2015 describes computer modeling of a ball-and-socket hub that enables teetering for three-bladed wind turbines. Such modeling showed that a three-bladed turbine with a teetering hub provides very significant reductions in the bending loads applied to the main shaft in comparison to a three-bladed turbine with a rigid hub. This is because teetering largely eliminates the cyclic variations in torque that are present with rigid hubs. A lifetime fatigue study using a rainflow counting of multi-axial torque applied to the blade root showed that a three-bladed turbine with a teetering hub provides for an approximate six-fold reduction in lifetime blade damage in comparison to a three-bladed turbine with a rigid hub. The modeling also showed that a three-bladed turbine with a teetering hub provides significant benefits in comparison to a two-bladed turbine with a teetering hub. This is because addition of a third blade reduces all loads applied to the blades by one-third. The lifetime fatigue study showed a teetering, three-bladed turbine provided an approximate four-fold reduction in lifetime blade damage in comparison to a teetering, two-bladed wind turbine.
Subsequent computer modeling and finite element analysis performed on a teetering, three-bladed turbine with a ball-and-socket hub determined that the ball-and-socket teetering hub design would likely be limited to small wind turbine applications because it does not provide sufficient strength to withstand the loads applied by a very large wind turbine (5 MW). Additionally, the ball and socket design requires considerable mass and presents significant manufacturing challenges for large wind turbines. An additional disadvantage of the ball-and-socket hub design is that it provides limited options for improving the strength of the hub because all parts have to be incorporated into either the ball or socket.
In view of the foregoing, it would be desirable to provide a new design that is superior to the ball-and-socket hub design. Accordingly, there is a need for fluid-driven turbine hubs that enable teetering of three or more blades, that can be manufactured with reduced mass and cost, that are suitable for large turbine-generator systems (e.g., capable of generating approximately 100 kW or more), and that overcome limitations associated with conventional devices.